Bengals defensive end Antwan Odom said on his Twitter account that he has tested positive for a banned substance and he faces a suspension.
Odom acknowledged that his case is under appeal. If the appeal is rejected by the league, he could miss up to four games.
There is no timetable for when a resolution might happen.
"To my fans and teammates: Please don't believe the rumor," Odom said in a tweet. "Yes I tested positive for a banned substance but it wasn't steroids or PEDs. More details to come but for now it's a league issue that's under appeal. Just know that I would never cheat to gain an edge in this game that I love. Don't lose your faith in me and please don't pass judgement based on vicious rumors until all the facts are known. Thank you for all your support I really need it now. I love all of you."
A league spokesman said earlier Thursday that the NFL had no suspension to announce when specifically asked about Odom.
Odom is coming off surgery on a torn Achilles tendon. At the time of his injury, he was tied for the 2009 league lead in sacks with eight in six games.
John Clayton is a senior writer for ESPN.com.